[Python-Dev] Rationale for sum()'s design?

Alex Martelli aleaxit at yahoo.com
Tue Mar 15 23:01:48 CET 2005

On Mar 15, 2005, at 01:16, Tim Peters wrote:

> [Eric Nieuwland]
>>> Perhaps the second argument should not be optional to emphasise this.
>>> After all, there's much more to sum() than numbers.
> [Greg Ewing]
>> I think practicality beats purity here. Using it on
>> numbers is surely an extremely common case.
> I'd personally be delighted if sum() never worked on anything other
> than numbers.  That makes it easy to understand, easy to document,
> easy to remember, obvious at first sight, and straightforward to
> implement.  Everything a framework isn't, but it's not a bad thing to
> have *something* that actually means exactly what it looks like it
> says <0.5 wink>.

I'm reasonably often using sum on lists of datetime.timedelta 
instances, "durations", which ARE summable just like numbers even 
though they aren't numbers.  I believe everything else for which I've 
used sum in production code DOES come under the general concept of 
"numbers", in particular X+0 == X.  Unfortunately, this equation 
doesn't hold when X is a timedelta, as X+0 raises an exception then.


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